Arrays

“Lists” of Variables Sometimes, it becomes necessary to order variables in a list: Temperature on Sept 1 = 90 Temperature on Sept 2 = 87 Temperature on Sept 3 = 84 in order to spot trends.

Lists of variables All of the variables and values are of the same type – integers, strings, whatever. Their order is important. Their place in the list is important May be shortened as… Temp1 = 90 Temp2 = 87 Temp3 = 84

Track the order as a separate variable Called an “index”: Temp[1] = 90 Temp[2] = 87 Temp[3] = 84 x = 2 Temp[ x ] = ? Name [ index ] = value…. index can change!

Arrays A difficult concept at first : int z[5]; Means: z[0], z[1], z[2], z[3], z[4] are separate integer variables Starts at 0. An array with 5 elements is indexed from 0 to 4. in Java: int z[ ] = { 0, 0, 0, 0, 0 }; Or int z[ ] = new int[5];

Array example int x; int z[ ] = { 0, 1, 2, 3, 4, 5, 6, 1, 1, 1 }; // z[4]= ? 1! for (x = 0; x < 10; x = x+1) { z[x] = x; } What is the value of z[4]? 4! in fact… z[0] = 0 z[1] = 1 z[2] = 2... to z[9] = 9

Arrays do not have to be numbers int x; String myArray[ ] = new String[5]; myArray[0] = ”hello"; myArray[1] = ”we"; myArray[2] = ”have"; myArray[3] = ”a quiz"; myArray[4] = ”next week"; for (x=0; x <= 4; x=x+1 ) { System.out.println( myArray[x] ); }

What are arrays good for? – They are confusing just to read… int a[ ] = new int[ 5 ]; EmployeeInfo[ 1021 ] Temperature[ x ]

What are arrays good for? – They are hard to track… “context awareness” is difficult What’s going on in this code? int x, RunningTotal = 0; int Sum[ ] = new int[ 5 ]; for (x = 0; x <= 4; x++) { RunningTotal = RunningTotal + x Sum[ x ] = RunningTotal; }

Sum[0] = 0 Sum[1] = Sum[2] = Sum[3] = Sum[4] = = 10

What are arrays good for? Think of the use of lists, that are –ordinal (ordered in sequence) –item positions are important –lists can be anything: strings, names, numbers, ages, costs, addresses

Polygons! int xPoints[ ] = { 55, 67, 109, 73, 83, 55, 27, 37, 1, 43 }; int yPoints[ ] = { 0, 36, 36, 54, 96, 72, 96, 54, 36, 36 }; Polygon star = new Polygon(xPoints, yPoints, 10);

Using three arrays to keep information String lastName[ ] = new String[ 5 ]; String firstName[ ] = new String[ 5 ]; int age[ ] = new int[ 5 ]; e.g. lastName[ 0 ] = “Truman”; firstName[ 0 ] = “Harry”; age[ 0 ] = 175;

array items are correlated by index

continued lastName[ 1 ] = “Garcia”; firstName[ 1 ] = “Jerry”; age[ 1 ] = 55; lastName[ 2 ] = “Smith”; firstName[ 2 ] = “John”; age[ 2 ] = 24;

continued lastName[ 3 ] = “Mouse”; firstName[ 3 ] = “Mickey”; age[ 3 ] = 75; lastName[ 4 ] = “Buckley”; firstName[ 4 ] = “Mike”; age[ 4 ] = 39;

array items are correlated by index record table field

What is this? Information that is grouped by index Kept in arrays e.g. lastName[1], firstName[1], and age[1] go together to form one profile for person #1 Is a DATABASE. We’re creating our own database.

printing out the info – note index int index = 0; userInput = JOptionPane.showInputDialog(“Enter index: “); index = Integer.parseInt( userInput ); if (index >=0) && (index <=4) { System.out.println(“ Item: ” + index ); System.out.println(“First Name: ” + firstName[ index ]) ; System.out.println(“Last Name: ” + lastName[ index ] ); System.out.println(“Age: ” + age[ index ] ); }

just imagine… Each array contains thousands of items More arrays to hold soc. sec. #, birthdate, pay scale, years of service Program ways to enter data as well as display. A full-scale Database Management program.

Changing array values - age public static void changeAge( int indexToChange ) { int newAge; userInput = JOptionPane.showInputDialog(“Enter age: “); newAge = Integer.parseInt( userInput ); age[ indexToChange ] = newAge; } note: array index is a parameter passed into the method nothing is returned, array is changed directly within the method (why? because the arrays are public)

Changing array values – first name public static void changeFirstName( int indexToChange ) { userInput = JOptionPane.showInputDialog(“Enter First: “); firstName[ indexToChange ] = userInput; }

Changing array values – last name public static void changeLastName( int indexToChange ) { userInput = JOptionPane.showInputDialog(“Enter Last: “); lastName[ indexToChange ] = userInput; }

printing out the info – note index // get user request, exit after user enters 4 or greater int index = 0; do { userInput = JOptionPane.showInputDialog(“Enter index: “); index = Integer.parseInt( userInput ); System.out.println(“First Name: ” + firstName[ index ]) ; System.out.println(“Last Name: ” + lastName[ index ] ); System.out.println(“Age: ” + age[ index ] + “for index ” + x); } while (index < 4 )